Full-duplex speakerphones using present signal processing technology have problems with acoustic echo cancellation. Since full-duplex speakerphones should be able to operate in a variety of environments, they are designed to accommodate environments having high levels of acoustic echo. The acoustic echo may be a direct-path echo from the loudspeaker to the microphone, or may be a reflected echo, where the delay of the reflected echo may be several hundred milliseconds, depending on the size of the room, location of objects, etc. The direct-path echo typically has a larger signal strength than a reflected echo. Thus, the result of direct-path echo may be a large reduction in dynamic range, as well as an annoying howling from the loudspeaker.
Full-duplex speakerphones typically implement adaptive echo cancellation which uses very large AFIR (adaptive finite impulse response) filters for canceling acoustic echo. Adaptive echo cancellers are usually very effective at reducing reflected, or delayed, acoustic echo because the echo signal strength is smaller, and has been changed from the original audio signal. However, when there is a direct-path echo and several reflected echoes, the direct-path echo has a larger signal strength than the signal strength of any of the reflected echoes. The adaptive echo canceller attempts to cancel the strongest signal among the echoed signals. Therefore, the adaptive echo canceller may tend to ignore the weaker reflected echoes when strong direct-path echoes are present.
The typical solution to direct-path echo reduction is to isolate the microphone from the loudspeaker by as much effective distance as possible. In expensive teleconferencing equipment, the loudspeaker may be mounted on a wall or a ceiling, for example, and the microphone may by placed on a table which is a sufficient distance from the loudspeaker to reduce the effects of direct-path echo. In other speakerphone systems where the microphone and the loudspeaker are in the same housing, or enclosure, sound damping materials may be used to attempt to isolate the microphone from the loudspeaker.